Large mode area (LMA) fibers are widely used in the fabrication of optical fiber-based elements such as high power lasers and amplifiers. The increase in mode area with respect to conventional fibers offer several benefits, such as reduced signal impairment (associated with nonlinearities), increased overlap of a pump signal with the gain medium in an amplifier, and increased energy storage capacity. However, increasing the mode area of these fibers results in them becoming “multi-moded”, where multiple spatial modes can be excited through discrete or distributed scattering events along the length of the fiber. These multiple spatial modes interfere with the fundamental signal mode, altering the center of mass position of the beam and increasing the minimum focused spot size (M2). It is therefore desirable to remove the higher order spatial modes present within fiber systems formed of LMA fiber.
Much of the current research in high power fiber lasers is devoted to engineering complex LMA fiber configurations so that they effectively support only the fundamental mode. The index profile and/or dopant profile of LMA fiber is designed so that the fiber exhibits differential gain for the fundamental mode or, alternatively, differential loss for the higher order modes. Coiling and/or tapering LMA fibers have previously been used to strip away higher order modes. However, tight bending of LMA fiber shifts the mode away from the fiber axis and reduces the mode area, which increases nonlinearity. It also impacts the degree of overlap between the signal and the doped core region of the fiber thereby reducing gain. Fiber taper lengths are relatively long (e.g., a few centimeters) which results in increased nonlinearity arising from propagation in a small mode area, and the tapered fiber section is fragile and requires special packaging.